This invention relates to a gas turbine engine having a centrifugal compressor and immediately downstream of it a centrifugal diffusor from which the compressor air flow is deflected into an axial direction via a substantially 90 degree elbow and is further decelerated in an axial-flow stator cascade arranged upstream of the combustion chamber. Arranged immediately downstream of the centrifugal compressor, is a main bearing of the gas generator. This main bearing is supplied from the outside through the vanes of the axial-flow stator cascade.
Highly-stressed centrifugal compressors, especially centrifugal compressors of gas turbine engines, are normally fitted with two stators for maximum conversion of the dynamic pressure downstream of the impeller into static pressure by deceleration of the flow. Following a first centrifugal stator cascade often fitted with wedge-shaped vanes, the flow is deflected, especially with gas turbine engines, through 90.degree. and is then decelerated in a second axial-flow cascade. The airfoil sections used in this second cascade roughly corresponds to those of an outlet stator cascade with highly-stressed axial-flow compressors.
If the general design of the engine calls for a main bearing immediately downstream of the centrifugal compressor--which is often recommended for reasons of efficiency and performance--all bearing supply lines (fresh oil, return oil, sealing air and possibly bearing chamber venting) must necessarily be routed through the flow duct. When this is the case it is generally impossible to route these supply lines through the radially wetted portion of the stator with its thin, wedge-shaped vanes. When the bearing is supplied through freely exposed lines running through the flow duct downstream of the axial-flow stator cascade, these will cause irregular flow, normally to the great detriment of component assemblies downstream of the compressor, as perhaps the combustion chamber of a gas turbine engine. When the bearing is supplied through the vanes of the axial-flow portion of the stator, the form of the airfoil sections of the cascade is generally less than ideal. This is aggravated by the fact that it invariably takes a group of blades to serve any one function, as e.g. for draining the oil, because each cascade section has only little free cross-sectional area available. At the same time there is an unfavorable ratio of circumference to cross-sectional area of the partial ducts, which is a considerable disadvantage especially for the oil ducts (high heat transfer, oil heating). This compels considerable complexity of design when splitting the various streams into a number of partial streams. This effort is duplicated when the various partial streams are subsequently gathered into the respective main stream.
A broad object of the present invention is to improve conventional gas turbine engines of this generic category such that the main bearing downstream of the centrifugal compressor is optimally supplied while ensuring proper aerodynamic conditions for the axia-flow stator cascade.
Another object of the present invention is to provide an improved gas turbine engine of the foregoing character, which is substantially simple in construction and may be economically fabricated.
A further object of the present invention is to provided an arrangement, as described, which has a substantially long operating life.